This invention relates generally to local area communication networks, and, more particularly, to such networks employing a unidirectional bus or ring to connect together a number of data communication devices, such as computers. In some types of communications systems, it is desirable to be able to transmit data from one computer to selected other computers. Providing a dedicated communication link between each unique pair of computers is obviously a costly and impractical solution, and various configurations of bus or ring networks have been developed to allow any computer connected to the network to "broadcast" a message to one or more of the other connected computers. The configurations for such networks available prior to this invention suffer from various drawbacks that limit their performance or increase their complexity and cost.
In a ring network, each of the computer stations is connected to transmission medium in the form of a closed ring. In this and the other systems to be discussed the transmission medium may be a coaxial cable to carry a modulated carrier signal at a selected radio frequency, or may be an optical communications link carrying a modulated light beam. The principles of the invention apply equally well to both types of systems, but for very high-speed communication optical transmission may be the preferred embodiment of the invention.
Transmission onto a ring network is always made in the same direction. Typically, when a station is ready to transmit data it waits until it senses a passing "token", an artifact of the data moving around the ring, that indicates that transmission of more data is permissible. The station then transmits its "packet" of data.
Typical ring networks have been discussed in the following technical publications: M. V. Wilkes et al., "The Cambridge Digital Communication Ring", Proc. Local Comm. Network Symposium, May 1979, pp. 47-61; D. C. Loomis, "Ring Communication Protocols," Univ. of California , Dept. of Information & Computer Science, Irvine, CA, Tech. Rep. 26, Jan. 1973; and M. T. Liu et al., "Message Communication Protocol and Operation System Design for the Distributed Loop Computer Network "DLCN", Proc. 4th Annu. Symp. Computer Architecture, pp. 193-200, Mar. 1977.
The basic problem with closed-ring networks is that each station must remove packets addressed to it or transmitted by it. Otherwise, the packets will circulate endlessly. Examining each packet to determine if removal is appropriate takes processing time at each station, and delays progress of data packets around the ring. This propagation delay increases linearly with the number of stations in the network, and represents a significant disadvantage of ring systems in general.
A significant development in the field of bus networks was a system known as Ethernet, first described by R. M. Metcalfe et al. in a paper entitled "Ethernet: Distributed Packet Switching for Local Switching Networks," Comm. of the ACM, July 1976, pp. 395-404. Its fiber optics version, Fibernet, is described by E. G. Rawson and R. M. Metcalfe in "Fibernet: Multimode Optical Fibers for Local Computer Networks," IEEE Trans. on Communications, July 1978, pp. 983-90. Ethernet and Fibernet have the principal disadvantage that a station may have to wait for an indefinite time to transmit a packet of data, because of possible repeated collisions with other packets accessing the bus simultaneously. In the worst case, if a collision is encountered upon each transmission attempt, the network delay time is infinite. This possibility of an unbounded network delay renders Ethernet and Fibernet unsuitable for real-time data communication. Moreover, as the data rate of the transmission medium increases, the efficiency (or maximal throughput) of these systems decreases.
Two further developments, using a unidirectional bus, are known as Express-net and C-net, both of which are discussed in the detailed description that follows in this specification. Express-net is described in a paper presented by L. Fratta et al., entitled "The Express-Net: A Local Area Communication Network Integrating Voice and Data," at the International Conference on Data Communication Systems: Performance and Applications, Paris, Sept. 1981. Briefly, when an Express-net station is ready to transmit a packet it waits until it senses a passing "train" of data packets, then adds its own packet to the end of the train. A data train is a series of data packets, each originating from a station, and is led by a "locomotive," which is simply a burst of an unmodulated carrier signal. This will be discussed in more detail below. Express-net has a relatively high efficiency and a relatively low bounded network delay, making it suitable for real-time communication. However, the superposition of the locomotives from the stations may result in a large signal dynamic range which makes the receiver design difficult, especially for the applications in optical networks. Moreover, when starting the system from a completely idle condition a special "cold start" procedure is to be followed.
The C-net configuration, also to be discussed in more detail, avoids the necessity of a cold start procedure, and somewhat simplifies the communication protocols of Express-net. However, the maximum network delay of C-net is almost double that of Express-net.
Another problem in both Express-net and C-net arises from their specific protocols. There is an outbound bus along which data packets are transmitted, the end of this bus being folded back to form an inbound bus, which is monitored by receivers at each of the stations. The receivers of the stations have to be linked in sequence along the inbound bus, to sequentially detect the end-of-train condition, which is used to coordinate the subsequent packet transmissions. The received signal strength is therefore progressively attenuated, especially in the case of networks using optical fibers, as a data train passes successive receivers. To avoid this difficulty, additional station equipment is needed to boost the signal at each station along the inbound bus.
It will be appreciated from the foregoing that there is still a need for a communication network having high efficiency, low and bounded network delay, and relative simplicity of protocol. Ideally, such a network should also be readily adaptable for use in networks of various sizes, while retaining its high efficiency and providing relatively uniform received signal strengths at each station's receiver. The present invention is directed to these ends.